The present invention pertains to testing microelectronic circuits such as semiconductor integrated circuits.
Electronic circuits, including semiconductor integrated microelectronic circuits, are tested during and after the manufacturing process. Testing verifies that the circuit has been accurately fabricated and is functioning properly.
Test circuits are sometimes included in the design of an integrated circuit. These test circuits are designed to generate particular responses to known inputs. In addition, test points are established in the circuit at which signals in the circuit may be monitored during testing.
Electrical paths are provided to the test circuits and to the test points so that the signals to be monitored may be detected. In addition, in certain circumstances, the tester may be required to supply specific known test inputs to the test circuit or to a particular point in the circuit so that a particular function may be tested.
Each of these input paths and output paths generally requires a contact point at which external test apparatus may be connected. Each such contact point includes a pad on the integrated circuit to which a test probe may be attached (for testing at the die stage), and an external pin (for testing after packaging).
A conductive path, or trace, is designed into the integrated circuit to conduct the signal to be observed from the point on the circuit on which it is generated to a point at which a contact pad and/or pin can be provided. Generally these contact pads are placed at the perimeter of the integrated circuit. A trace also may be necessary to provide a path for an input test signal from a contact pad to a particular point in the circuit.
Contact points such as pads on the integrated circuit or pins in the packaged device for device testability are readily available when the tester is required only to test the overall device by supplying input signals to the regular device input, and by observing the output signals at the regular device output. The tester can simply contact the regular signal input and output points of the device to perform such a test. However, the tester may be required to observe a point other than the regular device input and output. Each additional point to which test contact is desired requires an additional contact pad and pin, and an additional trace from the circuit point to be observed to the contact pad.
The additional contact point or points, and the traces from the contact points to the circuit points to be tested, complicate the design of integrated circuits. Sometimes the designer is forced to decide between testability and efficient design. For example, there may not be sufficient room for an additional contact pad on the integrated circuit to provide a test contact. Or, there may not be an available pin on the package. In other circumstances, a conductive trace from a point in the circuit to a contact pad may interfere with other aspects of the circuit design.
Once testing of the microelectronic device is completed, the test circuit and the electrical connections to the test circuit usually have no useful function. However, they continue to occupy space in the device. On the integrated circuit, the pads for the input and output pins for the test circuit consume valuable chip "real estate." In addition, physical size often limits the number of input and output pins that may be included on a packaged chip. Pins on the packaged device needed only for testing may displace pins that could be used for other purposes related to the operation of the integrated circuit for its intended purpose by its ultimate user.